fixing issue #563

[pchampin]

No description provided.

[joernhees]

Returning a Literal(0) where we can't find anything seems wrong to me. Could we return None instead?

sample query on virtuoso

[gromgull]

Sample of nothing should be an error:

http://www.w3.org/TR/sparql11-query/#defn_aggSample

Sample({}) = error

[joernhees]

yes, the problem is that there are various meanings of "error"... IMO they don't mean to remove the whole result row / cancel the whole query, but just the one binding within the row, as shown here:
http://www.w3.org/TR/sparql11-query/#aggregateExample2

So if we encounter such an error, i think it's correct to somehow return the row with a placeholder indicating an unbound variable (which i thought None was), rather than removing the result row or simply cancelling the whole query.

At least that's what Virtuoso does in the sample query above, so it must be true ;)

[joernhees]

@gromgull except for the Literal(0) this seems legit to me, any thoughts?

[pchampin]

I'm terribly sorry about this mess with Literal(0). I totally agree that this seems wrong, I did it because I couldn't find the part of the spec where it was specified (thx @gromgull for pointing it out), and I thought that Virtuoso and Corese both did that. Obviously, they do not, they both return NULL instead, I still don't know how I messed up to get this impression :-/